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Radiators: classic, modern, and custom

by Daniel Stern (part 3 of a series)

The radiator is a liquid-to-air heat exchanger: hot coolant flows through the radiator's metal tubes, which are connected to thin metal fins. Air flows across the fins, carrying away the heat.


The parts of the radiator are the core (the matrix of tubes and fins), the inlet and outlet tanks at each end of the core, and the attachment brackets. Cars with automatic transmissions also have a transmission fluid cooler inside the outlet tank, which runs at lower temperature than the inlet tank because the coolant in the outlet tank has already been cooled. This is a liquid-to-liquid heat exchanger: hot transmission fluid flows in and its heat is transferred via the metal fluid cooler into the also-hot-but-relatively-cool coolant. (Many modern cars have auxiliary transmission fluid coolers as well.)

For a long time, radiators were made out of brass and copper with steel brackets; the whole thing was soldered together. In the late 1980s, the auto industry began moving toward smaller aluminum radiators, often with crimped-and-gasketed plastic tanks. These work fine, but once they're terminally clogged or the tank gasket lets go, they're done; a leaky joint can't be soldered up as with the old copper-brass radiators. That's more or less just as well; today's radiator shop is most likely to be a place where you can take your (recent-model) car to get the radiator "boiled out" (steeped in a hot cleaning solution) or a new off-the-shelf radiator swapped in, but not an actual hands-on repair job.


The cooling capacity (heat rejection) of a radiator depends on how much coolant it puts in simultaneous contact with the primary heat transfer surface-the interior of the tubes. This generally depends on the size and number of tubes; for years it has been common parlance to refer to a radiator by the number of rows of tubes it contains. A standard light-duty factory radiator might have one or two rows of tubes, thus be called a "1-row" or a "2-row" radiator. A heavy-duty radiator might be a 3-row item, and a super heavy duty radiator might be a 4-row.

As the row count grows, so does the thickness of the radiator core, and this limits the possibilities for increasing cooling system capacity.

In the 1980s, "high efficiency" radiator cores became available; they have smaller tubes-about 3/8" major diameter instead of 1/2"-so more of them can fit in a given core thickness, and they have more and smaller fins to increase the metal-to-air heat transfer surface. A 3-row high-efficiency core is not much thicker than a 2-row standard core, opening up some options for upgrade. But smaller tubes are more easily clogged, and restrict flow more-that's not so good. Fortunately, it's also possible to go the other direction and get a good result; now there are aluminum radiator cores available with very large tubes that may be a whole inch in major diameter. For a core of any given width and height, a single row of 1" tubes can provide better heat rejection than two or three rows of smaller tubes.

A radiator may be a downflow design (coolant enters the top tank, moves downward through vertical tubes, and exits from the bottom tank), or it may be a crossflow design (coolant enters one side tank, moves sideways through horizontal tubes, and exits from the other-side tank). Neither is inherently superior, but downflow radiators tend to be taller for any given capacity. Chrysler stuck with downflow radiators much longer than most other American automakers, but crossflow designs have been essentially universal for decades now because their lower profile suits vehicles that have plenty of width available at the front, but not much height. Nearly all rear-drive Mopars came with downflow radiators, and there's usually no reason to switch to a crossflow type (not that there's any functional reason to avoid a crossflow radiator).


It's growing more difficult to find an off-the-shelf correct direct replacement radiator of acceptable quality-or at all-for the older Mopar applications. A custom-built radiator is one solution. I had such a radiator built by Randy Pettey for a challenging application: my 1962 Dodge Lancer has an aluminum 225 engine (coolant capacity 31% smaller than the iron-block engine), a cramped engine compartment (no room for a thick radiator core) and air conditioning (extra heat load). It's got a single row of big 1" tubes and a high fin count. It wasn't cheap, but the craftsmanship is beautiful, it weighs very little, and it's a terrific part.

Most rear-drive Mopars have more radiator space than my Lancer, so the options are less constrained. Universal crossflow radiators are readily available in a variety of sizes and quality levels (if you buy cheap Chinese, you get cheap Chinese!). They don't look anything like the originals, they tend to require cutting and drilling to fit, and it can be a challenge coming up with hoses that fit neatly-you probably won't find curved and formed radiator hoses that work; such hoses are made to connect a particular engine in a particular car to a radiator with its inlet and outlet in particular positions. But the universal radiators can cost quite a bit less than a custom build.

2: The surprisingly wide range of radiator caps <> 4: Choosing coolant: old, new, and water-free


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